Article
Physics, Multidisciplinary
Noeleen Naidoo, Sunil D. Maharaj, Keshlan S. Govinder
Summary: A model of a radiating star undergoing gravitational collapse is constructed by studying the boundary condition and equation of state. New classes of exact solutions with acceleration, expansion and shear in the presence of the cosmological constant are obtained. The range of the equation of state parameters allows the solution to be interpreted as barotropic matter stars or dark energy stars. The model is physically reasonable and satisfies the energy conditions.
Article
Physics, Particles & Fields
Noeleen Naidoo, Sunil D. Maharaj, Keshlan S. Govinder
Summary: The relationship between radiating stars and Riccati equations in general relativity is examined, considering a general matter distribution including the electromagnetic field and the cosmological constant. A generalized transformation for describing the gravitational potentials of a spherically symmetric relativistic gravitating fluid is introduced, leading to a new Riccati equation at the surface of the radiating star. Exact solutions satisfying the boundary condition are obtained, and the gravitational potentials are explicitly given. Some consistency conditions are reduced to Bernoulli equations, allowing for exact solutions. It is also shown that reducing the order enables the boundary condition to be expressed as a first order equation using the generalized transformation. Solutions obtained using the generalized transformation also satisfy a linear equation of state.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Pretty N. Khambule, Rituparno Goswami, Sunil D. Maharaj
Summary: The study demonstrates how geometrical quantities of different spacetimes are related on a matching surface with inherited symmetry, requiring matching Gaussian curvature to constrain thermodynamic quantities for smooth matching.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
Andreas Konstantinou, Sharon M. Morsink
Summary: Rotation affects a neutron star's mass and radius, but not its equation of state. Measurements of rotating neutron stars can be used to infer the mass and radius of nonrotating neutron stars. The changes in polar and equatorial radii exhibit symmetry.
ASTROPHYSICAL JOURNAL
(2022)
Article
Physics, Particles & Fields
Daniel Suarez-Urango, Justo Ospino, Hector Hernandez, Luis A. Nunez
Summary: This paper explored the physical acceptability conditions for anisotropic matter configurations in General Relativity and found that polytropes based on the total energy density are more viable than those with baryonic density, and small positive local anisotropies produce acceptable models.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
G. Lugones, A. G. Grunfeld
Summary: This study revisits the quark-mass density-dependent model and shows that thermodynamic inconsistencies can be solved by formulating the model in the canonical ensemble instead of the grand canonical ensemble. The introduction of a bag term in the pressure, due to the density dependence of quark masses, naturally explains quark confinement. The chemical potential also gains a new term due to quark repulsive interactions.
Article
Astronomy & Astrophysics
J. R. Stone, V Dexheimer, P. A. M. Guichon, A. W. Thomas, S. Typel
Summary: A new equation of state for cold and hot hyperonic matter has been developed in the context of the quark-meson-coupling model, yielding results compatible with existing nuclear physics constraints and astrophysical observations. The study explores the presence of hyperons in proto-neutron stars and neutron stars, as well as the nucleon-hyperon phase transition. Results show a correlation between hyperon presence and increasing temperature and density, with implications for understanding neutron star merger remnants.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2021)
Article
Mechanics
Stefano Boccelli, James G. McDonald
Summary: This work introduces a limitation on the minimum value of energy in a relativistic gas with a non-zero heat flux. The limitation arises from the non-negativity of the particle distribution function and is found by solving the Hamburger moment problem. The results show that in order for the distribution function to be non-negative, the energy of the gas must exceed a minimum threshold, and the heat flux has a maximum value determined by the energy and the pressure tensor.
Article
Astronomy & Astrophysics
Yu-Ying He, Xin-Jian Wen
Summary: The thermodynamics of finite size quark matter in the quasiparticle model is studied. An effective bag function is used to consider the medium effect to the confinement. Analytic surface tensions in strong magnetic fields are obtained using the multiple reflection expansion, showing anisotropic structures with splitting between longitudinal and transverse surface tensions. The anisotropy of surface tension can be enhanced by higher magnetic fields, and the surface tension is modified by an additional term related to the bag function.
Article
Astronomy & Astrophysics
Panagiotis Iosif, Nikolaos Stergioulas
Summary: The emergence of novel differential rotation laws has provided a way to construct equilibrium models that resemble the rotational profile of binary neutron star merger remnants in numerical simulations. The models have been successfully constructed using a 4-parameter differential rotation law and three tabulated equations of state. The results show that the chosen rotation law can accurately reproduce the threshold mass for prompt collapse to a black hole, with a relative difference of only about 1% compared to numerical simulations. Furthermore, the compactness of the equilibrium models at the threshold mass may be correlated with the compactness of maximum-mass non-rotating models. The study also finds a relatively slowly rotating inner region in the remnants, where the angular velocity is mostly due to frame dragging.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Simon Blouin, Antoine Bedard, Pier-Emmanuel Tremblay
Summary: The Gaia colour-magnitude diagram shows a clear separation between hydrogen-atmosphere white dwarfs and helium-atmosphere white dwarfs during the cooling process. However, the magnitudes of pure-helium atmospheres are too similar to those of pure-hydrogen atmospheres to explain this bifurcation. To explain the observed split, it has been suggested that there must be trace amounts of hydrogen and/or metals in the helium-dominated atmospheres of hydrogen-deficient white dwarfs. This study aims to provide a comprehensive explanation by considering population synthesis simulations, model atmospheres, and evolutionary calculations along with element transport in white dwarf envelopes. The simulations successfully reproduce the bifurcation and highlight the importance of convective dredge-up of carbon in explaining the observations.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
J. Sedaghat, S. M. Zebarjad, G. H. Bordbar, B. Eslam Panah
Summary: We have performed a leading order perturbative calculation to obtain the equation of state (EoS) of strange quark matter (SQM) at zero temperature under a magnetic field B = 10(18) G. The obtained EoS is then used to calculate the maximum gravitational mass and corresponding radius of a magnetized strange quark star (SQS). The results show a maximum gravitational mass over three times the solar mass, and the validity of isotropic structure calculations for SQS is investigated.
Article
Astronomy & Astrophysics
Kamal Krishna Nath, Ritam Mallick, Sagnik Chatterjee
Summary: Recently, it has been discovered that neutron stars (and quark stars) follow certain universality, known as the I-Love-Q relations, meaning that knowledge of any one observable can provide insights into the structural and macro properties of these compact astrophysical objects. This universality is due to the low sensitivity of the relations to the equation of state (EoS) of matter. In this study, the consistency of universal relations for a generic family of equations of state was investigated, and it was found that the I-Love-Q trio holds relatively well up to a tolerance limit of 5% fractional percentage error, even with significant variations in the EoS.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Astronomy & Astrophysics
Ankit Kumar, H. C. Das, S. K. Patra
Summary: Motivated by theoretical studies on efficient dark matter capture by neutron stars, this research explores the indirect effects of captured dark matter on the cooling mechanism of a neutron star. The study shows that varying dark matter momentum greatly modifies neutrino emissivity and investigates specific heat and thermal conductivity of a dark matter admixed star to explore cooling wave propagation within the star.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Astronomy & Astrophysics
Hristijan Kochankovski, Angels Ramos, Laura Tolos
Summary: The FSU2H equation-of-state model is extended to finite temperature and it is found that temperature has a stronger effect on thermodynamical observables and the composition of neutron star cores when considering hyperonic degrees of freedom. The widely used Gamma law is not appropriate to reproduce the true thermal effects, especially when hyperons become abundant in the neutron star core. Simple parametrizations of the thermal index for nucleonic and hyperonic beta-stable neutrino-free matter are provided to make finite temperature equations of state easily accessible.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2022)
Article
Physics, Mathematical
Chevarra Hansraj, Rituparno Goswami, Sunil D. D. Maharaj
Summary: In this paper, we study conformally symmetric generalized Vaidya spacetimes with composite null dust and null string matter distribution using a decomposition method. Our analysis reveals that the geometric variables related to both time-like and preferred space-like congruences are completely determined by the conformal vector and conformal factor. This uniqueness arises from the specific matter distribution of the generalized Vaidya configuration. Additionally, we demonstrate that a proper conformal Killing vector cannot be admitted in the case of pure null dust (or Vaidya) spacetime.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Physics, Multidisciplinary
Noeleen Naidoo, Sunil D. Maharaj, Keshlan S. Govinder
Summary: A model of a radiating star undergoing gravitational collapse is constructed by studying the boundary condition and equation of state. New classes of exact solutions with acceleration, expansion and shear in the presence of the cosmological constant are obtained. The range of the equation of state parameters allows the solution to be interpreted as barotropic matter stars or dark energy stars. The model is physically reasonable and satisfies the energy conditions.
Article
Astronomy & Astrophysics
Byron P. Brassel, Sunil D. Maharaj, Rituparno Goswami
Summary: The junction conditions for a higher dimensional spherically symmetric charged and anisotropic static star in Einstein-Gauss-Bonnet (EGB) gravity with nonvanishing cosmological constant are derived. It is shown that the generalised matching conditions are satisfied across the timelike boundary hypersurface of zero thickness in EGB gravity. The validity of Israel-Darmois conditions is a sufficient condition for the matching. Therefore, a complete stellar model can be generated in EGB gravity. The interior matches to the exterior higher dimensional charged Boulware-Deser spacetime with cosmological constant. The vanishing of barotropic radial pressure at the boundary of the star is also consistent with general relativity.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Mathematics, Interdisciplinary Applications
Milliward Maliyoni, Holly D. Gaff, Keshlan S. Govinder, Faraimunashe Chirove
Summary: Spatial heterogeneity and migration of hosts and ticks affect the spread, extinction and persistence of tick-borne diseases. In this paper, we use a stochastic differential equations model to investigate the impact of white-tailed deer and lone star ticks migration on the dynamics of tick-borne diseases. We derive a general formula for computing the basic reproduction number for all patches and show that disease extinction can be increased by controlling or prohibiting the movement of hosts and ticks.
FRONTIERS IN APPLIED MATHEMATICS AND STATISTICS
(2023)
Article
Astronomy & Astrophysics
Shavani Naicker, Sunil D. Maharaj, Byron P. Brassel
Summary: In this study, we investigated the influence of higher order curvature terms on the static configuration of a charged dust distribution in EGB gravity. We found exact solutions to the governing equations using special functions, series, and polynomials. The results show that the charged dust model behaves well in the center region and can generate dust spheres. The higher order curvature terms have a distinct influence on the dynamics of charged dust and gravitational behavior compared to general relativity.
GENERAL RELATIVITY AND GRAVITATION
(2023)
Article
Physics, Particles & Fields
Shavani Naicker, Sunil D. Maharaj, Byron P. Brassel
Summary: In this paper, the field equations for a charged gravitating perfect fluid in Einstein-Gauss-Bonnet gravity are derived in all spacetime dimensions. The static and spherically symmetric spacetime leads to a charged condition of pressure isotropy, which is a second-order Abel differential equation. It is shown that this equation can be simplified to a first-order, nonlinear canonical differential equation in higher dimensions. The canonical form allows for an exact solution generating algorithm, resulting in implicit solutions by choosing potentials and the electromagnetic field. An exact solution to the canonical equation is found, which corresponds to the neutral model previously discovered. Additionally, three new classes of solutions are obtained by placing constraints on the canonical differential equation, without specifying gravitational potentials and electromagnetic field. When the canonical form is not applicable, other classes of exact solutions are presented using elementary and special functions (the Heun confluent functions).
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
C. Hansraj, R. Goswami, S. D. Maharaj
Summary: In this paper, the Kerr spacetime is analyzed using a semi-tetrad decomposition. The 1+1+2 covariant method is applied to describe the geometry of the Kerr spacetime outside the horizon. A comparison is made between observers in the Killing frame and the ZAMO frame, resulting in different geometrical quantities and equations. This novel approach provides new insights into the Kerr spacetime geometry in a transparent manner.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Sunil D. Maharaj, Noeleen Naidoo, Gareth Amery, Keshlan S. Govinder
Summary: The Karmarkar embedding condition in different spherically symmetrical metrics is studied using Lie symmetries. The study extends recent results by investigating the Lie symmetries for conformally flat and shear-free metrics. Additionally, the Lie symmetries for geodesic metrics and general spherical spacetimes are obtained for the first time. The study also demonstrates that the Karmarkar condition can be used to produce an embeddable relativistic radiating star with a barotropic equation of state via Lie symmetries.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Noeleen Naidoo, Sunil D. Maharaj, Keshlan S. Govinder
Summary: The objective of this study is to investigate spherically symmetric radiating stars undergoing gravitational collapse, in higher dimensional general relativity, inclusive of acceleration, expansion, shear, an electromagnetic field and a cosmological constant. Two approaches are studied to obtain exact solutions to the boundary condition with/without a linear equation state. Transformations that map the boundary condition into a new Riccati equation are investigated to obtain new exact models. The importance of these transformations in reducing the order of the boundary condition and obtaining new solutions is shown.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Multidisciplinary
Alberto K. Mathias, Jefta M. Sunzu, Sunil D. Maharaj, Jason M. Mkenyeleye
Summary: Exact solutions for charged relativistic anisotropic stars are generated using the Karmarkar condition with Einstein-Maxwell field equations and a linear equation of state. The Bannerji and Durgapal transformation is employed to transform the nonlinear differential equations and field equations. The embedding approach provides a relationship between gravitational potentials, helping to solve and integrate the field equations. The detailed physical analysis shows that the model satisfies necessary conditions such as stability, equilibrium, energy conditions, and the mass-radius relationship.
PRAMANA-JOURNAL OF PHYSICS
(2023)
Article
Physics, Multidisciplinary
J. W. Jape, S. D. Maharaj, J. M. Sunzu, J. M. Mkenyeleye
Summary: We generate charged anisotropic exact models with a conformal Killing vector for static spacetime, which satisfies various physical requirements and regains some known solutions.
INDIAN JOURNAL OF PHYSICS
(2023)
Article
Physics, Multidisciplinary
J. W. Jape, J. M. Sunzu, S. D. Maharaj, J. M. Mkenyeleye
Summary: Exact solutions of charged anisotropic fluid spheres with a linear equation of state are generated by incorporating conformal symmetry and an equation of state. These solutions satisfy several important physical conditions, including smooth matching, compatibility with observations, and stability requirement.
INDIAN JOURNAL OF PHYSICS
(2023)
